To examine the factor structure of temperament in 5-10-year-olds with Williams syndrome, an exploratory factor analysis was conducted on the responses of parents of 192 children on the children's behavior questionnaire. Four factors were identified. Two corresponded to factors reported for typically developing children: effortful control and extraversion/surgency and two corresponded to the temperament constructs of withdrawal/inhibition and irritability/frustration and activity, observed in typically developing infants. Parents of 109 of the 192 participants also completed the anxiety disorders interview schedule, parent version. Children with an anxiety disorder other than specific phobia differed significantly from children without an anxiety disorder on all factors except extraversion/surgency. Children with attention deficit hyperactivity disorder (ADHD) differed significantly from children without ADHD on effortful control and extraversion/surgency. (Contains 2 tables, 1 figure and 1 footnote.)